Hong Kong babies have a low risk of contracting German measles, despite two cases of the highly contagious virus in infants born to mainland mothers in the city, the health minister said.

Food and Health Secretary Dr York Chow Yat-ngok said yesterday that both infants were in stable condition, but long-term medical treatment might follow.

'Hongkongers are usually vaccinated against the virus when they are young,' Chow said. 'Around 90 per cent of local girls and women have immunity to the disease, and the children they carry will naturally gain immunity from the mother. So the risk of infection to other babies from [the two cases] is very low.'

There were 87 cases of infection last year, but no baby has had the congenital condition since 2008, when it was detected in one infant born to a mainland mother.

'As more mainland women that have not had a proper antenatal examination give birth in Hong Kong, it is natural to see more babies [with the congenital illness],' Chow said.

He said he was worried about the condition of the two newborns. One, a boy, had pneumonia, a heart defect and other organ abnormalities at birth, and the other, a girl, had heart and brain abnormalities; both might confront other serious problems as they developed.

'A lot of time and resources are needed to treat these two children,' he said.

The boy was transferred to Queen Elizabeth Hospital on January 13, and the girl was transferred to the same hospital on February 8.

The symptoms of German measles, also known as rubella, are usually mild.

But congenital rubella syndrome, caused by women who contract the virus during the first three months of pregnancy, can be passed onto the fetus, resulting deafness, eye lesions, heart defects and mental retardation in the child.

Chow urged all expectant mothers to have a thorough antenatal examination.